The present invention relates to methods and apparatuses for detecting chattering in cold rolling mills. In particular, the present invention relates to a method and an apparatus suitable for detecting chattering, which occurs during cold rolling of a steel strips in a cold rolling mill.
It has been conventionally known that a vibration phenomenon of a rolling mill called chattering occurs in some cases during cold rolling of a strip (for example xe2x80x9cAtsuen Hyakuwaxe2x80x9d (various stories about rolling) by Suzuki in xe2x80x9cKikai no Kenkyu (Studies of Machines)xe2x80x9d published by Yokendo, Vol. 48, No. 5, pp. 583-588). When the amplitude of the vibration is small, lateral stripes formed at a certain pitch in a direction perpendicular to the rolling direction are merely observed on both front and back sides of the rolled strip. When the amplitude of the vibration is large, however, the thickness of the rolled sheet periodically varies. In the case of a significant variation in the thickness, the minimum thickness of the strip becomes even a half or less of the maximum thickness. When the amplitude of the vibration is more significant, the rapture of the strip may occur due to a further increased variation in the thickness.
FIG. 1 shows an example of observed thickness offset (xcex94t) of a cold-rolled strip which is rolled when chattering occurred. Periodical thickness variations occur in the longitudinal direction (L) of rolling. Among portions having such thickness variations, segments (hatched portions in the drawing) outside the tolerance limit are discarded as failure portions in the subsequent step or in an intermediate step before the product is shipped. That is, a decrease in yield and an extra maintenance operation may cause deterioration of production cost.
When the rupture of the strip occurs, the rolling line must be unavoidably stopped for a long time, resulting in significant deterioration of production efficiency.
Thus, the detection of the chattering phenomena is important. In many cases of chattering, initial vibrations with small amplitudes develop into vibrations with larger amplitudes within 2 to 3 seconds. Thus, in daily operations, the initiation of the chattering must be highly sensitively and rapidly detected to perform any countermeasure, for example, deceleration of the rolling speed.
Various methods and apparatuses have been proposed for detecting chattering.
For example, Japanese Examined Patent Application Publication No. 5-87325 discloses a method for detecting the occurrence of chattering when a difference in the thicknesses which are simultaneously observed at two or more points in the longitudinal direction of the material to be rolled exceeds a predetermined value. The measurement of the thickness is performed at an interval which is substantially the half the pitch of the generated variation in the thickness. Herein, it is known that the variation in the thickness of the rolled strip due to chattering during cold rolling is 1 to several xcexcm and the period of the variation is several tens of msec. Thus, the thicknessmeter must have high detecting resolution and a short response time. Thicknessmeters satisfying these two requirements are considerably expensive. According to this method, two radiation thicknessmeters being expensive apparatuses must be proximately installed at a position for originally installing one apparatus. Thus this method has a problem of increased facility cost.
Japanese Unexamined Patent Application Publication No. 8-141612 discloses a method for detecting chattering using detecting signals from a vibration sensor provided in a rolling mill. The detecting signals are processed using a filter having transmission characteristics which are set based on each operational condition of the rolling mill.
Japanese Examined Patent Application Publication No. 6-35004 discloses a method for detecting chattering using signals obtained by filtering the output from a vibration velocity sensor which is mounted in a housing of a cold rolling mill. The filter transmits only vibrations in a natural frequency range of the rolling mill.
Japanese Unexamined Patent Application Publication No. 8-108205 discloses a method in which vibration parameters of the rolling mill based on the observed data and rolling parameters of the rolling mill are subjected to a frequency analysis. When a frequency component which is an integer multiple of the fundamental frequency exceeds a predetermined value, the occurrence of chattering is determined. The vibration parameters of the rolling mill are detected during the operation using vibration detectors which are installed at least at one position of the rolling mill. The vibration parameters, which are detected and analyzed, are a vibration displacement, a vibration velocity, and vibration acceleration at each position. The rolling parameters are a tension, a rolling torque, and a rolling speed of the rolling mill. The fundamental frequency is obtained by calculating the natural frequency of the mill, and inherent vibration frequencies which are generated by interlocking of gears, failure of a bearing, unsuccessful coupling between a spindle and a roll, and flaws of a roll.
In any of the above conventional technologies, the detection of chattering is performed based on detected signals from vibration sensors at one or more positions of the rolling mills. These sensors, however, detect the vibrations due to the mechanisms of the rolling mill, in addition to the vibrations due to the chattering. That is, when the frequency components of vibrations of the mechanisms of the rolling mill include in the frequency range for the frequency components of the chattering, erroneous detection of the chattering occurs.
In the conventional technologies, outputs from a plurality of vibration sensors and the frequencies of the rolling parameters must be analyzed at high speeds. Thus, the scale and the cost of the apparatus are unavoidably increased. Moreover, the vibration based on the abnormal mechanical system in the rolling mill and the vibrations of the resulting rolling parameters are merely requirements regarding the factors for generating the chattering. Thus, the occurrence of chattering due to other factors may be missed. On the other hand, an abnormal mechanical system before chattering or vibrations of the rolling parameters may lead erroneous detection of chattering. As a countermeasure against this problem, for example, Japanese Unexamined Patent Application Publication No. 8-108205 discloses a method for momentarily analyzing or calculating the frequencies based on the vibrations of individual components and the outputs of the rolling parameters of the rolling machine and the theoretical vibration based on the abnormal mechanical system. In this method, however, a vibration sensor must be installed in a mill housing or in the vicinity thereof. In this case, the vibration sensor is placed in adverse environments, for example, oil in the mill and roll-cooling water. Such adverse environments result in severe deterioration of the vibration sensor and the replacement of the vibration sensor is a bother.
On the other hand, the applicant proposed a method by an acoustic measurement, which is different from the above methods, in Japanese Unexamined Patent Application Publication No. 60-137512.
In general, vibration of a substance vibrates the air in the vicinity thereof and propagates the vibration as sound. The acoustic measurement is generally performed by detecting the pressure fluctuation of the air at a predetermined position. An acoustic sensor detects and signalizes this pressure fluctuation and the resulting signals are acoustic signals. A microphone is a typical acoustic sensor and outputs the acoustic signals as electrical signals. The sound has frequency components whereas the acoustic sensor exhibits frequency characteristics, such as a detectable frequency range and frequency-dependent sensitivity. Thus, the acoustic signals change depending on the acoustic sensor used. The time variation of the acoustic signals forms an acoustic waveform. The acoustic waveform contains high-frequency vibration components having short periods. Acoustic signals after eliminating the high-frequency vibration components are specially called sound intensity, which is often used as a parameter representing acoustic characteristics. The high-frequency vibration components are eliminated by, for example, calculating the effective value of the acoustic signal (for example, square integrated value within a given time interval) or a peak amplitude of the acoustic signal within a given time interval. Various values derived from the acoustic measurement such as the acoustic intensity are acoustic parameters.
The above proposal discloses a method in which a tone inherent in the chattering during rolling of the cold rolling mill is converted into an electrical signal and the occurrence of the chattering is detected when the magnitude of the electrical signal exceeds a predetermined value. The first embodiment of this method is shown in FIG. 2. During rolling a material 8 to be rolled, tones in the vicinity of individual rolling stands 11 in a tandem cold rolling mill 10 are converted into electrical signals using a microphone 14 as an acoustic sensor. The electrical signals enter a band pass filter 22 so as to transmit only signals in a chattering frequency range. The outputs from the band pass filter are rectified for a predetermined time interval to output an integrated value. The integrated value is input into a comparator circuit (CMP) 29. If the input signal exceeds a predetermined value, the comparator circuit generates a chattering-detecting signal. The detecting signal is input into a driving circuit 31 to operate an acoustic apparatus 32. Moreover, another embodiment is shown in FIG. 3. The microphone 14, the comparator circuit 29 outputting the chattering-occurrence signals when the input signal exceeds the predetermined value, and the subsequences are substantially the same as those in the first embodiment. The electrical signals from the microphone are analyzed in a frequency analysis circuit (FA) 42, and the output from the frequency analysis circuit enters a band pass filter 22 to extract frequency components inherent in the chattering. The output signal from the band pass filter is input into the comparator circuit 29.
In this method, no acoustic sensor is placed in the mill housing, and the number of the sensor is one. Thus, this method has an advantage of easy maintenance compared to the use of the vibration sensor.
When a noise containing frequency components similar to those of the chattering is generated at other places in the rolling plant, erroneous detection of the chattering tends to occur, because a signal is distinguished only by the frequency components from the sound detected by the acoustic sensor.
In the first embodiment of Japanese Unexamined Patent Application Publication No. 60-137512, the output waveform is still an AC waveform. Even if the waveform is integrated for a given time interval, the integrated value becomes substantially zero. Thus, this method cannot detect a phenomenon of increasing amplitude of the frequency components inherent in the chattering. In the second embodiment, the frequency analysis circuit generally does not have a function for outputting waveform signals, and thus, it is difficult to obtain information on the occurrence of chattering from the band pass filter.
The standard for judgement in the conventional technologies is to detect whether or not the frequency components inherent in the occurrence of the chattering is are contained in the observed vibration waveform or the observed acoustic waveform. The present inventors have discovered by long-term intensive experiments at operation sites that impulsive vibrational phenomena generated at the interior and the exterior of the rolling mill are also detected together with the vibrational phenomenon generated by rolling when the vibration waveform and the acoustic waveform are measured in the vicinity of the rolling mill during the rolling operation. Since these impulsive vibrations generally contain frequency components ranging from low frequencies to high frequencies, these impulsive vibrations are erroneously detected as chattering in some cases in the conventional technologies.
The inventors have intensively repeated the measurements in the production sites and have discovered that one of such noise phenomena is pulsed sound. This impulsive vibrational state is shown in FIG. 4. FIG. 4(a) shows a time variation of an acoustic signal (A) in an acoustic waveform which is observed in the vicinity of the cold rolling mill, wherein the acoustic signal depends on the properties of the acoustic sensor used and has an arbitrary unit. FIG. 4(b) shows a time variation of an output (VB) from the band pass filter containing only the frequency components inherent in the chattering, based on the input of the acoustic signal. FIG. 4(c) shows a time variation of a rectified value (VA) of the output from the band pass filter. FIG. 4(d) shows a time variation of the output (VC) from a comparator device which submits an alarm output when the rectified waveform exceeds a threshold value, and FIG. 4(e) shows a time variation of the velocity (v) of the material to be rolled. FIG. 4(a) includes pulses at positions indicated by arrows, and the pulses sound alarms, as shown in FIG. 4(d). However, as shown in FIG. 4(e), the rolling velocity does not change. That is, the rolling state is normal without chattering. Accordingly, when a pulsed acoustic wave occurs, the conventional apparatus sounds an alarm regardless of a normal rolling state.
In order to eliminate such a pulsed waveform as noise, a method for smoothing by the moving average of the amplitude of the waveform has been conventionally used. When the time interval for the moving average is larger than the duration width of the pulsed noise, the peak value of the noise is reduced in response thereto. However, a large width of the moving average causes a delayed response time in detection of the occurrence of the chattering, although the noise is reduced. That is, the occurrence of the chattering cannot be rapidly detected. As a result, the operation action tends to be delayed, resulting in increased chattering failures. Moreover, the operational treatment is not in time, and the rolled material may be ruptured.
Accordingly, no method for exactly and rapidly detecting the occurrence of the chattering has been established.
The present invention has been accomplished in order to establish a method for detecting the occurrence of chattering exactly and rapidly. That is, an object is to detect the occurrence of chattering during the cold rolling operation correctly using a simple configuration, without effects of noise due to factors other than the rolling operation and impulsive vibration applied to facilities including rolling mills and auxiliary rolls between stands.
Accordingly, the present invention relates to a method for detecting chattering of a cold rolling mill using a plurality of acoustic parameters derived from a sound measured in the vicinity of the cold rolling mill during rolling. The acoustic parameters are as follows; Acoustic intensities of a frequency range characteristic of the occurrence of chattering and frequency ranges of N-th harmonic (frequency ranges having upper and lower limits corresponding to N times of the upper and lower limit of, the frequency range characteristic of the occurrence of chattering), the peak frequency in the acoustic frequency component distribution, the resonance factor, and the peak intensity. The same parameter may be measured and calculated at different types of timing as a plurality of parameters. Also, the present invention relates to an acoustic sensor, a circuit for calculating a plurality of acoustic parameters from acoustic signals in the sensor output, and an apparatus for detecting chattering of a cold rolling mill using the plurality of acoustic parameters and for submitting a signal.
An example of the acoustic waveform observed when the chattering occurs is shown in FIG. 5. It is well known that the acoustic waveform is nearly equal to a sine wave when the time axis is enlarged. In the same observation, a frequency component distribution of an acoustic signal at a certain time is shown FIG. 6. The acoustic signal component at a certain frequency is represented by Af having an arbitrary unit. Peaks are intensively observed in the vicinity of certain frequencies. According to the description by T. Tamiya et al.: xe2x80x9cAnalysis of chattering phenomenon in cold rollingxe2x80x9d (Proc., Intl., Conf., on Steel Rolling, 1980, Vol. 2), the chattering phenomenon is explained as a resonance phenomenon of a coupled vibration system of a rolling mill frame and a rolling roll. When the sound due to vibration of the rolling mill is observed at a time of the occurrence of the chattering, peaks appear in a narrow band in the vicinity of the chattering frequency in the frequency distribution of the acoustic signal. The acoustic signal in regions other than the chattering frequency is small.
In contrast, an example of an acoustic waveform containing impulsive vibration occurring at the interior and the exterior of the rolling mill is shown in FIG. 7. A frequency component distribution of an acoustic signal at a certain time in the same measurement is shown in FIG. 8. In FIG. 8, peaks are observed over a wide range, unlike in FIG. 6. The acoustic signal other than the peak frequency is substantially the same level. When an acoustic signal which is larger than a predetermined value is detected, one due to chattering and one due to others such as an impulsive sound can be discriminated. Thus, only the occurrence of the chattering can be detected.
For example, the waveform discrimination can be quantified with a resonance factor Q. FIG. 9 exhibits a frequency component distribution of an acoustic signal. The peak frequency at the maximum acoustic signal frequency component is set to be f0, and frequencies having an acoustic signal frequency component of 1/{square root over (2)} at the upper and lower sides of the peak frequency are set to be fl and fh. The resonance factor Q is defined as follows:
Q=f0/(fhxe2x88x92f1)xe2x80x83xe2x80x83(1)
The sharpness of the sound resonance can be quantified by the resonance factor Q. This value can detect the occurrence of the chattering.
The present invention is based on this principle.